Prior to performing work on electrical installation/equipment, workers are required to verify that the equipment is in an electrically safe state. Until proven otherwise, one must assume that the equipment is energized and take all necessary precautions, including utilizing appropriate personal protective equipment (PPE). One part of the verification of an electrically safe work condition involves a test for the absence of voltage. This test is performed by a trained and qualified electrician using an adequately rated voltage tester, usually a portable voltmeter or multi-meter. The electrician first tests his meter on a known, energized source to ensure it is working properly. He/she then verifies that voltage is absent in the electrical equipment by metering phase-to-phase and phase-to-ground. Finally, he re-tests his meter on a known, energized source to ensure it is still functioning properly and wasn't damaged during the test. Although voltage verification is an NFPA 70E requirement and considered a best practice, the test itself still presents a hazard because workers are exposed to energized circuits and conductors when using the voltage tester during the live portions of the test.
A permanent installed device that is able to detect the presence and verify the absence of primary (single- or multi-phase AC or DC) voltage and positively indicate the status of voltage in a particular electrical compartment would be useful for this type of application.
One of the key steps to verifying the absence of a signal is to perform a check to verify that the device/tester can detect a known voltage unit (test the tester). One other step is to verifying the unit is actually measuring the signal and has not unknowingly been disconnected (connectivity test).